1. Field of the Invention
The present invention relates to a novel method of producing a die for forming fisheye steps on a lens and, in particular, a method which can improve the appearance of the fisheye steps by making a connection line, formed from the boundary lines between the adjacent fisheye steps, assume a line approximating a straight line.
2. Description of the Background Art
The designing of lenses of vehicular lamps inevitably includes designing of steps which have various shapes depending on the requirements from the light distribution control and the design. For example, fisheye steps, which are formed as an unevenness on the back surface of a lens, are used in a tail lamp etc., of an automobile.
FIG. 14 shows an example a of an outer lens, and is a formed product made of a synthetic resin material.
The outer lens a has a shape consisting of a plate-like portion b and a portion c that is continuous with the portion b and is curved and twisted as reaching along the longitudinal direction to one end. With this shape, the lens surface matches the external shape of a vehicle body.
As shown in FIG. 15, a large number of fisheye steps d, d, . . . are formed on the back surface (light incident surface) of the outer lens a so as to be arranged vertically and horizontally with each convex step occupying an approximately rectangular unit section e, e, . . . .
Since the vertical and horizontal cross-sections of the fisheye steps d, d, . . . generally have a shape of a circular arc, the fisheye steps d, d, . . . refract the light directly coming from a light source (not shown) or the light reflected by a reflection surface and deflect such light vertically and horizontally, to thereby produce a distribution of light somewhat diffused vertically and horizontally.
Therefore, when the plate-like portion b of the outer lens a is viewed from the front side, the vertical and horizontal boundary lines of the steps are seen through the lens as an orthogonal grid-like pattern; however, the curved portion c is designed such that the boundary lines of the steps form a non-orthogonal grid in accordance with a curvature of this portion.
In this design, in order to obtain a good appearance of the lines seen through the outer lens a, it is important to connect the step boundaries extending in the longitudinal direction of the outer lens a so that they form a line extending as closely in the horizontal direction as possible, and to connect the step boundaries extending in the vertical direction so that they form a line as close to a straight line as possible.
FIGS. 16(a) and 16(b) conceptually show a method of machining a die for the outer lens a, and FIGS. 19(a) and 19(b) show the part of the die that serves to form the fisheye steps on the curved portion c of the outer lens a.
The die machining process is automated using an NC machine tool or a machining center. NC machining data, which includes such information as machining conditions and movement instructions for a cutting tool, is generated based on a lens shape model obtained by a CAD system, and a die is machined by controlling the cutting tool in accordance with the NC machining data.
In order to form convex shapes of the fisheye steps, the die material is cut by an edged tool g called a ball end mill to produce concave portions f, f, . . . that correspond to the intended convex shapes, as shown in FIG. 17. The movement of the ball end mill is controlled so that it , takes a locus h as shown in plan views of FIGS. 16(a) and 16(b), and generally rectangular (as projected to a plane) concave portions f, f, . . . are formed by repeating this cutting movement a plurality of times.
Reference character u in FIG. 16(a) represents a direction of longitudinal lines i, i, . . formed as the boundaries of the concave portions f, f, . . . , and character which is perpendicular to uv, represents a direction of lateral lines j, j, . . . formed as the boundaries of the concave portions f, f, . . .. A vector v.sub.-- I represents a movement direction of the end mill g with respect to a lateral line js on the start side, and a vector v.sub.-- O represents a movement direction of the end mill g with respect to a lateral line je on the end side.
It is noted that the terms "start" and "end" with respect to the lateral lines j, j, . . . are defined relative to the movement of the end mill g, and are employed for the convenience of description.
As shown in FIGS. 16(a) and 16(b), the end mill g is controlled so as to enter the concave portion f in parallel with the longitudinal lines i, i, . . . , proceed while cutting the die material, and exit also in parallel with the longitudinal lines i, i, . . . .
The cutting is performed so that the cutting depth of the concave portions f, f, . . . becomes larger as the position reaches the centers of the concave portions f, f, . . . and smaller as the position reaches the longitudinal and lateral boundary lines.
However, according to the above method, the adjacent lateral lines i, i, . . . do not join smoothly as shown in FIG. 19(b). As a result (i.e., by transfer), irregular lateral lines appear in the outer lens a of the formed product, and the appearance of the fisheye steps d', d', . . . are deteriorated as shown in FIG. 18.
This is due to the following reasons, with reference to FIGS. 16(a) and 16(b). If the concave portions f, f, . . . are formed such that a first concave portion coincides with a second concave portion by moving the first one in the v direction, the lateral lines are connected to form a straight line but also extend obliquely, i.e., in the v direction. To avoid this phenomenon in which the lateral lines gradually deviate from the horizontal direction, it is necessary to shift the concave portions f, f, . . . in the u direction little by little.
In the case of the fisheye steps formed on the plate-like portion b of the outer lens a, since their boundary lines assume an orthogonal grid form and the individual steps are separated as rectangular sections, the boundary lines are joined to form straight lines. However, in the case of the fisheye steps on the curved portion c of the outer lens a, the boundary lines intersect each other to assume a non-orthogonal grid and the individual steps are separated as parallelogram sections. Therefore, if the concave portions f, f, . . . are formed by the end mill g proceeding in parallel with, the longitudinal lines i, i, . . . (see FIGS. 16(a) and 16(b)), the lateral lines j, j, . . . do not join properly, and the boundary lines of the fisheye steps after the forming, particularly the lines extending in the horizontal direction, are zigzagged, as seen in FIG. 18.
As the streamlined vehicle body design becomes a common trend in view of its desired aerodynamic characteristics and the design of the vehicle body and what is called a slanted body is employed widely, lamps are required to be designed to match the vehicle bodies so designed. As a result, the zigzagged lateral lines of the fisheye steps will be more noticeable, particularly in the curved portion of the outer lens, deteriorating the appearance.
As a result, according to the above-described method, it takes much time and work to modify the die and the machining data, which is one of the causes of reducing the efficiency of operation.
To solve this problem, it may be assumed that the locus of the end mill in forming the generally parallelogram-like concave portions is not changed (i.e., remains the same as in FIGS. 16(a) and (16) and the end mill entering and exiting positions with respect to the respective concave portions are gradually shifted in the u direction. However, in order to perform continuous machining according to this method, the control of the end mill movement becomes complex. On the other hand, discontinuous machining with respect to the v direction will cause disadvantages such as an increased machining time.